1. Field of the Invention
This invention relates to a method and apparatus for controlling the gage of the material in a rolling mill, or more in particular to a method and apparatus to compensate for the eccentricity of rolls in a rolling mill.
2. Description of the Prior Art
Today, there is an increasing demand for high precision in the thickness of rolling mill products, and the automatic gage control system of gate meter type built around what is called BISRA-AGC (Automatic Gage Control developed by the British Iron & Steel Research Association) has made rapid progress.
This automatic gage control system of gage meter type is almost necessarily provided for controlling the thickness of the material in the rolling mill and arranged to control the desired gage hd, the roll gap S at no load, the rolling pressure P and mill constant Km in such a manner as to satisfy the equation EQU hd - (S + P/Km) = 0
In the automatic gage control system of the gage meter type, however, it is impossible to maintain the roll gap constant and therefore to achieve the objects of the gage control if there is an eccentricity of any of the rolls. In other words, the above-mentioned control system of gage meter type is such that any increase in the rolling pressure is considered to be caused by an increase in input gage and acts to reduce the no-load roll gap S. This ignores the fact that in the event of the roll gap being reduced by roll eccentricity, the rolling pressure is also increased, thus undesirably further reducing the rolling gap. Therefore, the elimination of the effect of the roll eccentricity is an important problem to be solved in the automatic gage control system of gage meter type.
Various methods for solving this problem have been suggested in the past. Many of them, however, are too complicated in construction or low in precision to be used widely, with the result that the gage is generally controlled in reliance upon the skill of the operator. In the simplest one of such methods, as an example, the automatic gage control system is so constructed that a filter for passing only a component of the roll eccentricity cycle fe is inserted in the feedback loop of the rolling pressure controlling circuit thereby to eliminate the component of the roll eccentricity cycle fe from the feedback signal, so that the eccentricity component does not affect the roll gap. This method, however, has the disadvantages that; (1) since the components of a frequency fe in the rolling pressure variation are regarded as roll eccentricity components and allowed to pass, the gage variation component of the same frequency fe as the roll eccentricity is passed, resulting in a larger gage variation without any correction; (2) what is called the resonance type filter which resonates with the roll eccentricity frequency fe allows to pass components near the resonance frequency band, thereby to pass not only the signals to be passed but other signals in the neighborhood thereof.
Another method which has been suggested in the past for gage control is based on a hypothetical position of occurrence and frequency of a roll eccentricity. In other words, it is assumed that the roll eccentricity occurs in the back-up rolls at the frequency of fe and the detected frequency component of the roll eccentricity is assumed to represent all the roll eccentricity. The detected waveform is subjected to Fourier analysis thereby to pick up only the roll eccentricity frequency component which is used to correct the roll eccentricity component in the gage control system.
Nevertheless, it is generally true that the disturbances in the roll system occur also in the work rolls, and, in a complex pattern including high frequencies. For this reason, the above-mentioned methods for detecting and correcting one roll eccentricity component only fails to attain a satisfactory accuracy.